Process for surface hardening steel articles



May 3, 1960 Filed Nov. 19, 1956 R. PRIBYL 2,935,433

PROCESS FOR SURFACE HARDENING STEEL ARTICLES 2 Sheets-Sheet 1 iinited States Patent PROCESS FOR SURFACE HARDENING STEEL ARTICLES Robert Pribyl, Linz (Danube), Austria, assignor to Vereinigte Osterreichische Eisenund Stahlwerlre Aktiengesellschaft, Linz (Danube), Austria, a firm Application November 19, 1956, Serial No. 623,211

Claims priority, application Austria November 25,1955 10 Claims. (Cl. 148-144) The present invention pertains to the process and apparatus for a high-grade surface hardening of steel articles which have to meet thetrequirements for heavy wear and high pressures per unit of area.

The known surface hardening processes for steel articles, employing gas-oxygen burners or electric heaters,

- have progressively increased in importance during the last 20 years. The feeding method and the rotating and feeding method have been most widely used.

The state of the prior art and the objects of the present invention will become apparent from the detailed description and the drawings which are set forth in the application.

In the drawings:

Figure 1 is a diagrammatic view of the process and apparatus of the prior art using a linear feeding method.

Figure 2 is a front elevation of a steel article in the process of hardening in accordance with the prior art employing a conventional combinationfeeding and rotating method. I

Figures 3 and 4 are graphical curves correlating the steps and conditions of the prior art as illustrated in Figure 1.

Figure 5 is a graph showing temperature and conditions along a longitudinal line of a steel hardened in accordance with the present invention.

Figure 6 is a diagram of the temperature conditions of a steel surface hardened in accordance with Figure 5, with an additional hardening step interposed.

Figure 7 is a diagrammatic view of a conventional steelhardening process employing a single'burner.

Figure 8 is a diagrammatic view of an improved process, described in Austrian Patent No. 174,627, employing two burners.

Figure 9 is a diagrammatical view of the process of the present invention.

Like reference characters denote similar or analogous parts, steps or conditions throughout the several views.

The performance of these methods of the prior art is diagrammatically shown in Figs. 1 and 2, respectively. In Fig. 1, 1 is the article to be hardened, e.g. a flat plate, shown in longitudinal section. 2 denotes the heat source, e.g. a fishtail burner, and 3 the quenching sprayer rigidly connected to the burner and spaced therefrom by the distance a. During the hardening operation the workpiece 1 is at rest and the burner and the quenching sprayer are moved at a constant speed in the direction v. Thus the hardening treatment proceeds along lines and is also referred to as a line hardening. Viewed in longitudinal section, the zone which has already been hardened, having the thickness s (depth of hardening), is identified in Figure l by fine hatching. In the rotating and feeding method shown in Fig. 2, 1 denotes the article to be hardened, e.g. a shaft held between centres. 2 is the heat source, e.g. a ring burner, and 3 is the quenching sprayer, which is connected to the burner and spaced therefrom by the fixed distance a. During the hardening operation the article 1 is rotated at a constant speed whereas the burner and sprayer are moved at a constant speed in the direction v so that the hardening progresses along a helix and is, therefore, referred to as helical hardening. In Fig. 2 the hardened surface is located to the left of quenching sprayer 3.

The said two hardening processes, however, are of limited application because the depth of hardening which is achievable is relatively small. Using the slowest feed, the work piece may be hardened to only a depth of about 6 mm., at most, by the fiamehardening process without causing a detrimental overheating of the workpieces.

A great advance consisted in a preheating of the workpiece to temperatures below the A0 point, more particularly to 500-550 deg. C. That preheating may be effected in a furnace or with the aid of a preheating burner, which is arranged preceding at a fixed distance the hardening burner. In Fig. 1 the preheating burner 4 has a distance b from the hardening burner. That process gives depths of hardening up to about 12 mm. by experience.

The Austrian patent specification No. 174,627 specifies preheating temperatures which lie above the critical temperatures, approximately on the same levels as the hardening temperatures. Experience has taught, however, that the depths of hardening thus achievable do not exceed about 15 mm. because a preheated piece when treated with the usual, strong hardening burners will obviously reach the hardening temperature sooner and the hardening process must be performed at a higher feed rate and, therefore, reduced time for the penetration of the heat applied in order to avoid detrimental overheating.

In the known feeding and rotating and feeding hardening treatments with gas-oxygen burners the depth of hardening which can be expected can be estimated from the zone of incandescence, i.e. the surface zone adjacent to the hardening burner which distinguishes by its colour of incandescence beginning, e.g., at about 650 deg. C., if the feed rate, the analysis and the thickness of the workpiece are also taken into account. For this reason that zone of incandescence is of great practical importance for the hardening operator. A narrow zone of incandescence is associated with a low depth of hardening whereas a wider zone of incandescence corresponds to a greater depth of hardening. In Fig. l the zone of incandescence, having the width B, is emphasized by cross-hatching.

All previously known oxyacetylene or electrical processes have the common feature that the highest surface temperature is produced in the range of action of the hardening device and that the part of the workpiece which lies before that range and has not yet been subjected to the progressive hardening treatment is at a medium temperature, which will in any case be lower than the hardening temperature. This means that in accordance with the narrow construction of the gas-oxygen hardening burners the zone of incandescence will also be relatively narrow. That narrow zone of incandescence results in a relatively thin hardened layer, which amounts, at most, to 15 mm. as has been mentioned hereinbefore.

This depth of hardening which can be achieved in accordance with the present state of the art is not sufficient, however, where the thickness of the workpiece is to be utilized to the largest possible extent in the case of heavy wear, e.g., in lining plates, armoured rods and beaters for disintegrating machines of crusher plates, grinding plates and grinding rolls for edge runners etc. Furthermore, it is not sufficient for tools which are subjected to a high pressure per unit of area and are reground several times during use, such as long blades for plate shears, rolls for straightening machines, working and backing rolls for rolling mills etc. Finally it is not sufficient to resist heavy bombardment. For this reason there is a great economic interest underlying the endeavour to achieve an increased depth of hardening.

In the process of surface hardening employing a preheating burner the temperature along a longitudinal line of the workpiece surface at a specific time follows a curve as shown in Fig. 3 or 4, which should be considered in conjunction with Fig. 1. According to Fig. 3 the preheating burner 4 raises the surface temperature on the short distance s very rapidly to the desired preheating temperature, e.g. to 500 deg. C. On the distance s which amounts in most cases to 400-600 mm. in practice, that temperature drops, e.g. to 400 deg. C. The hardening burner 2 raises the temperature on the distance s very rapidly to the hardening temperature, e.g., to 880 deg. C. On the relatively short distance s.;, which has in thicker workpieces a length of about 50 mm., at most, in practice, the temperature drops rapidly to the quenching temperature, e.g. to 830 deg. C. The quenching sprayer 3 reduces that temperature almost immediately to the temperature of the human body. The zone of incandescence, c-d, amounts to about 60 mm.

In the process described in the Austrian patent specification No. 174,627 mentioned hereinbefore the preheating burner heats the workpiece surface approximately to the hardening temperature, e.g., to 850 deg. C., but the width c-d of the zone of incandescence being dependent on the distance of the auxiliary burner 4 from the hardening burner 2, and the mean temperature of said zone, lying below the critical temperature of about 800 deg. c., are insuflicient for achieving a depth of penetration above mm.

According to the present invention the previous mode of working is entirely reversed. A procedure is adopted which is contrary to the previous basic concepts of hardening technology. The first step of heating of the workpiece raises the temperature thereof at its surface substantially above the A0 point, preferably to 950-1050 deg. C., and causes the workpiece to store a quantity of heat sufficient to maintain the surface of the workpiece at temperatures above the AC3 point in a zone of incandescence having a width of at least 250 mm. preferably of 400-600 mm., if desired by repeated heating until quenching is effected.

Surprisingly it has been found that even steels which are susceptible to cracking, e.g. the known cold roll steel having about 1% carbon and 2% chromium, will withstand that maximum temperature in the preheating zone without detrimental formation of coarse grain or cracks, particularly if the workpiece is preheated in a furnace in a manner known per se to some hundred degrees C. before the surface hardening treatment.

According to the invention the main emphasis lies on the preheating zone or the preheating burner, which was entirely absent or provided only as an auxiliary burner in the previously known processes. On the other hand the hardening burner, which was previously the essential means for effecting any hardening treatment, hecomes according to the invention an auxiliary burner, which is only used to retard or make up for temperature losses incurred in the preheating zone due to conduction and radiation and which may be entirely eliminated if the preheating by the preheating burner has been carried to a sufficiently high temperature of 1000 to 1150 C.

The typical temperature conditions obtained according to the invention along a longitudinal line of the workpiece surface are diagrammatically shown in Fig. 5. The relatively strong preheating burner 4 raises the workpiece surface very quickly, corresponding to the distance s' to the extremely high preheating temperature, e.g. to 1050 deg. C. It is obvious that the reduction of this high preheating temperature taking place along s, to the Ac temperature, e.g. to 800 deg. C., will take a relatively long time. Approximately at that point the relatively weak hardening burner 2 is applied at the distance b, in order to increase the surface temperature to the hardening temperature, e.g. to 880 deg. C.

As a result of the high preheating temperature the temperature will drop only very slowly along s so that the quenching sprayer 3 at the quenching point, e.g. at 830 deg. C., may be arranged at an extraordinarily large distance a from the hardening burner. Hence it follows a zone of incandescence of a width c--d', which previously was considered impossible, combined with an extremely great depth of hardening because the quantity of heat applied has sufiicient time for acting into the depth.

According to the invention a second weaker preheating burner 41: may be used which is disposed at a distance b behind the first preheating burner 4 (Fig. 6) and which raises the surface temperature, which has dropped approximately to the A0 temperature, again to about 1050 deg. C., at most. The reduction to the hardening temperature, e.g. 880 deg. C. is effected in that case even slower than in the preceding example shown in Fig. 5,

' owing to the larger heat content of the surface zone.

As a result the hardening burner 2 may be arranged at an even larger distance b". For this reason the reduction of the hardening temperature to the quenching temperature, e.g., 830 deg. C., is even slower, so that an increased spacing a" of the sprayer and a greatly widened zone e-f of incandescence is achieved, involving an increased depth of hardening.

It is also possible, of course, to increase the thickness of the hardened layer further by the use of a third preheating burner but this appears hardly necessary. The distance between the first preheating burner and the quenching sprayer is at least 250 mm. increased by mm. for each additional preheating burner. The invention can be applied to gasoxygen burner as well as to electric heating devices.

Surprisingly the tests with the aforementioned susceptible cold roll steel have shown that not only the preheating temperature but also the hardening and quenching temperature can be increased far beyond the known degree without detrimental, overheating or cracking of the article to be hardened. It is one of the established principles of hardening technology that the hardening temperature, i.e. the temperature to which the article to be hardened must be heated immediately before it is quenched, must be observed as accurately as possible. For this reason the standards specify limits of only 30 deg. C. for said temperature. In the normal hardening process the hardening temperature is 30-50 deg. C. over the Ac; point, whereas in the surface hardening process they are higher up to about 50 deg. C. according to the Stahl-Eisen-Werkstofiblatt No. 830- 850.

The quenching temperature corresponds theoretically to the hardening temperature but in practice is always somewhat lower because the workpiece heated to the hardening temperature will necessarily cool during the time which elapses until the quenching begins.

According to the invention the quenching temperature in surface hardening may be about 50-200 deg. C. above the usual value. In Fig. 6 these possibilities are indicated by the dotted portion of the curve. It is obvious that this will permit an even larger distance between the sprayer and the hardening burner and in consequence thereof a widened zone of incandescence e-g and an increased depth of hardening.

The conditions resulting in surface hardening treatments carried out according to the previously known processes and according to the process according to the invention are diagrammatically shown in Figs. 7 to 9, which represent the isotherms in the surface layer of the workpiece.

It is known that when quenching is effected at a rate exceeding the critical cooling rate only those zones will reach the full martensite hardness whose temperature agesegis's lies above the A point. in most cases approximately at 800 deg. C. An intermediate hardness having a bainite structure is then found down to that depth where the temperature is above the AC point. The depth of hardening extends to that point, lying in most cases about 750 deg. C. whereas the core zone which has not been heated above approximately 750 deg. C. does not undergo changes in structure and properties and thus remains unaffected by the surface hardening. In Figs. 7 to 9 the two critical isotherms, namely the 800 deg. C. isotherm limiting the zone of true martensite hardness and the 750 deg. C. isotherm limiting the depth of hardening are emphasized by thicker and dotted lines, respectively.

Fig. 7 shows the temperature pattern during the usual surface hardening employing no preheating. For instance, the plate 1 is hardened in the usual manner by a progressive movement of a unit comprising the hardening flame 2 and the quenching sprayer 3 in the direction v along the surface of the plate. The isotherms X represented in the drawing indicate the sudden temperature rise of the surface layer. Being characterized by a short chord and small vertical spacing they result in a narrow zone of incandescence and a thin hardened layer 5.

Fig. 8 illustrates the process according to the Austrian Patent No. 174,6 7, in which an auxiliary burner 4 for preheating is provided in front of the known hardening unit comprising the flame 2 and the quenching sprayer 3. The isotherms Y shown in the drawing are flatter (longer chords) and have a greater vertical spacing so that a larger depth of hardening layer 5 results.

In Fig. 9, which relates to the process according to the invention, the preheating burner is again denoted by 4. It supplies a relatively large quantity of heat so that the workpiece surface is heated, e.g., to 1050 deg. C. The hardening burner 2 has a relatively small output and delivers only such a quantity of heat as is required to maintain the desired hardening and quenching temperatures and to obtain the desired width of the zone of incandescence. The hardening burner must not cause a temperature increase to such an extent that the feed rate would have to be increased because that would reduce the depth of hardening. According to the invention the quenching sprayer 3 is spaced at least 250 mm., preferably 400-600 mm. from the preheating burner 4 so that there is sufficient time for the heat applied by the burners 2 and'4 to flow into the interior of the workpiece and a hardened layer 5 of a correspondingly large thickness will result upon quenching.

The isotherms Z distinguish by a much greater length of chord and vertical spacing and indicate that a much thicker hardened layer 5 is obtained.

In the hardening processes according to the invention depths of hardening of about 15-60 mm. are obtained, depending on the conditions chosen. These depths are sufficient for all cases previously discussed. In accord ance with the invention, large cold rolls are currently surface-hardened with one preheating burner and one hardening burner achieving a depth of hardening of about 40 mm. as well as a particularly gradual transition. The zone of incandescence reaches a Width of about 500 mm. In most practical cases a few experiments are suflicient to establish such a coordination of the preheating and hardening temperatures and of the feed rate that a detrimental overheating of the workpiece surface will be avoided.

The present invention extends the field of application of surface hardening substantially, to the limits of the technical requirements of the present time. The practice of the present invention saves high expenditures of the prior art.

The above description and illustration of the present invention is not limiting, since various changes may be made in the structures of the apparatus and methods described by those skilled in the art without departing from the spirit and the scope of the present invention and it is intended to cover all such changes and modifications.

What is claimed is:

1. Process for surface hardening steel articles, particularly cold rolls having a diameter exceeding about 300 mm. either by the feed method or the rotating and feed method and comprising the step of preheating the work piece surface by starting to heat the work piece surface to a temperature in the range of 950-1050 C. and by keeping the work piece surface in a zone of incandescence having a minimum width of 250 mm. and a temperature lying above the Ac point, the second step of heating the Work piece surface at the finishing end of the zone to hardening temperature within the range of SOD-880 C. and the third step of quenching the work piece surface by means of a sprayer.

2. Process for surface hardening steel articles, particularly cold rolls having a diameter exceeding about 300 mm. either by the feed method or the rotating and feed method and comprising the step of preheating the work piece surface by starting to heat the Work piece surface to a temperature in the range of 950-1050" C. and by keeping the work piece surface in a zone of incandescence having a width of 400-600 mm. and a temperature lying above the A0 point, the second step of heating the work piece surface at the finishing end of the zone to hardening temperature Within the range of 800-880 C. and the third step of quenching the Work piece surface by means of a sprayer.

3. Process for surface hardening steel articles, particularly cold rolls having a diameter exceeding about 300 mm. either by the feed method or the rotating and feed method and comprising the step of preheating the work piece surface by starting to heat the work piece surface to a temperature in the range of 950-l050 C. and by keeping the work piece surface in a zone of incandescence having a minimum width of 250 mm. and a temperature lying above the A0 point, the second step of heating the surface of the work piece to a hardening temperature of about 880-l080 C. and the third step of quenching the work piece from a surface temperature of about 800-1030 C.

4. Process for surface hardening steel articles, particularly cold rolls having a diameter exceeding about 300 mm. either by the feed method or the rotating and 'feed method and comprising the step of preheating the work piece surface by starting to heat the work piece surface to a temperature in the range of 950-l050 C. and by keeping the work piece surface in a zone of incandescence having a Width of 400-600 mm. and a temperature lying above the A0 point, the second step of heating the surface of the work piece to a hardening temperature of about 880-1080 C. and the third step of quenching the work piece from a surface temperature of about 800- 1030" C.

5. A method of hardening a steel surface comprising; at least one first step of heating the surface to a temperature substantially above the A0 point to at least 950 C.; a second step of permitting the surface to cool to a temperature below that of the first step, while allowing the work piece to store a quantity of heat sufficient to maintain said surface at a temperature above the A0 point; at least one third step of applying heat to the surface in order to increase the extent of the heated portion of the surface which is maintained at a temperature above the AC3 point; and a fourth step of quenching said surface.

6. A method of hardening a steel surface as claimed in claim 5, wherein the said temperature of the said first step is in the range of 950-1150 C.

7.A method of hardening a steel surface as claimed in claim 6, wherein the said temperature of the said second step is at least 830 C.

8. A method of hardening a steel surface as claimed in claim 5, wherein the said third step raises the tem- References Cited in the file of this patent perature Of the surface to about 880 C. UNITED STATES PATENTS 9. A method of steel hardening as claimed in claim 5, said distance of application of said fourth step from 2,254,307 Mott et a1 Sept- 1941. said first step being increased by 100 mm. for each addi- 5 2,295,272 9 Sept- 1942 tional third Step 2,590,546 Kmcard et a1. Mar. 25, 1952 10. A method of hardening a steel surface as claimed FOREIGN PATENTS in claim 5 wherein the distance of application of said 174,627 Austria Apt 25, 1953 fourth step is at least 250 mm. from said first step. 

1. PROCESS FOR SURFACE HARDENING STEEL ARTICLES, PARTICULARLY COLD ROLLS HAVING A DIAMETER EXCEEDING ABOUT 300 MM. EITHER BY THE FEED METHOD OR THE ROTATING AND FEED METHOD AND COMPRISING THE STEP OF PREHEATING THE WORK PIECE SURFACE BY STARTING TO HEAT THE WORK PIECE SURFACE TO A TEMPERATURE IN THE RANGE OF 950-1050*C. AND BY KEEPING THE WORK PIECE SURFACE IN A ZONE OF INCANDESCENCE HAVING A MINIMUM WIDTH OF 250 MM. AND A TEMPERATURE LYING ABOVE THE AC3 POINT, THE SECOND STEP OF HEATING THE WORK PIECE SURFACE AT THE FINISHING END OF THE ZONE TO HARDENING TEMPERATURE WITHIN THE RANGE OF 800-880*C. AND THE THIRD STEP OF QUENCHING THE WORK PIECE SURFACE BY MEANS OF A SPRAYER. 